1. Technical Field
The present invention relates to an image forming method and an image forming apparatus in which video signals corresponding to an image forming command are generated and then an image is formed. To be more specific, the apparatus forms a latent image over a latent image carrier, by means of an oscillation mirror scanning a modulated light beam that is obtained by modulating a light beam in accordance with the video signal along a main scanning direction.
2. Related Art
In an electrophotographic image forming apparatus, such as printers, copiers, facsimiles, and like, an image forming command is applied to a controller from an external apparatus such as a host computer, etc, in accordance with an image forming request from a user. Then, the controller converts the image forming command to a data format tailored for an operation command for an engine section. Further, while the engine section is controlled based on the data, an image corresponding to the image forming command is formed on a sheet (recording material), such as copy sheet, transfer sheet, paper and transparent OHP sheet, etc. This means that a video signal corresponding to the image forming command is generated based on a video clock signal and is output to an exposure unit in the engine section. Based on the video signal, a light source of an exposure unit is on/off controlled to emit a light beam, which is modulated in accordance with the video signal. The modulated light beam is scanned along a main scanning direction by a deflector of the exposure unit, thereby forming line latent images corresponding to the video signal over the latent image carrier of a photosensitive member and like. Then, these line latent images are developed with toner, whereby toner images are formed.
Use of a resonant oscillation mirror as a deflector is commonly proposed because of an increase in demand for a downsizing and a speed-enhancement of the deflectors (refer to JP-A-1-302317). In this apparatus, a frequency of drive signal (hereafter referred to as “drive frequency”) that is applied to the oscillation mirror is matched with a resonant oscillation frequency (resonant frequency) which is unique to an individual oscillation mirror. This makes the oscillation mirror resonate, and consequently relative large amplitude is obtained. Further, the light beam is emitted from the light source to the resonating oscillation mirror for the scanning of the light beam.
However, the resonant frequency of the oscillation mirror may fluctuate depending on the processing method of the oscillation mirror, the temperature around the oscillation mirror and the like. Such fluctuations will change the scanning speed of the light beam over the latent image carrier, thereby causing the latent image formed over the latent image carrier to shrink or expand along the main scanning direction, consequently reducing the image quality. To tackle this issue, the technology to control resonant frequency of an oscillation mirror to coincide with drive frequency has been proposed (refer to JP-A-2004-69731). Specifically, according to the invention described in this document, a thin film heater is mounted on the oscillation mirror as a heater element. Then, while detecting the swing angle of the oscillation mirror, the amount of electric current to the thin film heater is controlled to maximize the swing angle, thereby making the resonant frequency coincide with the drive frequency (Resonant Frequency Control). The swing angle of the oscillation mirror is detected in manner that a detection section such as a light detection element detects the light beam that scanned by the oscillation mirror.